Haven't had a chance to run your numbers Bookworm -- but a first observation is that you can't make a 180 in 30 seconds. At standard rate of turn --- +/- 21 AOB in an Arrow, 27 AOB in a fighter --- 180 requires one minute ---- 90 degrees to base = +/- 30 seconds.
You chose the circuit width (and therefore the turn diameter), not me! If you want to make a 180 degree standard rate turn from downwind on to final, you'll need to make the downwind about a mile from the runway. If you want to make it 3000 feet from the runway, you'll need to turn at greater than standard rate.